The present invention generally relates to an apparatus useful for positioning a light filter and, in particular, relates to such an apparatus useful for initializing a multi-segment light filter.
In many conventional analytical instruments, information about the sample under test is ascertained by passing a beam of light through the sample and subsequently detecting the relevant frequency spectrum of that light beam. As an example, an infrared spectrometer first passes a beam of light through the sample under test and then through a dispersive optical system, such as a grating, to separate a particular spectral band of interest. The light from the sample is monitored to gather characteristic information about the sample. The light from the sample is generally passed through a frequency sensitive filtering apparatus which removes light of frequencies other than that of interest. The filtered light is then directed onto a detector, commonly of the type which converts the light, based on the intensity thereof, to an electrical signal. The use of a light filtering apparatus is twofold; first, by rejecting light outside of a selected frequency band the signal to noise ratio of the light striking the detector is increased, and second, the character of the sample can be accurately determined by examining the light therefrom through filters having different spectral pass bands.
Hence, in an effort to accurately characterize a sample material, most infrared spectrometers include an apparatus for positioning various light filters, each having a different spectral pass band, in the path of the light beam emanating from the sample. In many instances, the filters are arranged in the form of a segmented wheel where each filter segment has a different spectral pass band. It is, of course, clearly necessary for the analyst or operator to know which filter segment is in the path of the light beam at all times. This information is usually acquired by initially positioning the filter wheel to a known position and thereafter monitoring the controlled rotation thereof. In addition, to ensure that the maximum amount of the light beam to be transmitted through the filter to the detector, it is desirable not only to select the proper filter segment but also to align that segment so as to achieve the maximum light transmission therethrough. Therefore, since the initial positioning of the segmented wheel is used as a basis for all other position setting thereof, it is critical that the initialization of the filter wheel be accurate.
Presently, the conventional instruments rely upon mechanical mechanisms to initially position the filter wheel. For example, in one apparatus, a tab extends from the filter wheel such that as the wheel rotates, the tab activates a switch as it passes thereby. The switching causes the wheel to be braked to position the selected filter between the light beam from the sample and the detector. However, such a mechanical activating system is subject to inaccuracies due to the inherent minor variations in the reaction times of the components due to age, friction, ambient condition or the like.